There are two different aspects in this question:

International merchadise trade statistics is concerned with goods physically crossing the border and it does not matter who buys and sell. For example if the UN buys food in Malaysia which is used for emergency aid and shipped to another country then Malaysia would record an export to this country and this other country would record an import.

The story is different if for example the UN buys furniture for its own offices or buys emergency food supplies or other goods and stores it for future use in its own territorial enclaves (which is probably in fact not really feasible as territorial enclaves are not designed for such purposes). In this case the goods physically enter UN territory. - Some countries have a special code for trade with international/ supranational  organisations, some countries might include this trade in a rest categorie and others might simple not specifically identifying it and simple record it as trade with the country in which the international/ supranatinal organisation is located.